Screen structure for cathode ray tubes



Nov. 25, 1958 M. SADOWSKY ETAL 2,862,130

SCREEN STRUCTURE FOR CATHODE RAY TUBES Filed June 26, 1953 I. R0 WW 8 C 3 ar: 2 MHM T H0 0 0 .6 WM 0 R H F 0 w .p J R R m mm mm M MM m 1 J0 5 X m I m 2 m 0 Z 2 2 L L L A A m mw W 0 .JP 5 0 m M .C .C I. 5 R 2 R M a H INVENTORS 7776/61? .SHDOd/J/f/ R/(HHRD 6. zmggmek BY United States Patent SCREEN STRUCTURE FOR CATHGDE RAY TUBES Meier Sadowsiry, Elkins Park, and Richard E. Waggener, Philadeiphia, Pa., assignors to Phiico Corporation, Philadelphia, Pa, a corporation of Pennsylvania Application June 26, 1953, Serial No. 364,398

26 Claims. (Cl. 31392) The presentdisclosure, which is a continuation-in-part of our copending U. S. patent application Serial No. 246,690, filed September 14,1951, relates to screen structures for cathode ray tubes which are adapted to produce visual and/or electrical indications of beam impingernent upon predetermined portions thereof and also to television systems which include such cathode ray tubes and which operate to produce visible images on their screen structures.

In suchtelevision systems the receiver is supplied With a signal which has successive portions representative of information respecting different primary colors of minute elements of the televised scene. This signal is applied to the beam intensity control grid electrode of the image reproducing cathode ray tube where it serves to control the intensity of the beam as it scans the screen structure of the tube. The screen structure which is most suitable for the reproduction of the intelligence represented by this received signal consists of a large number of minute fluorescent elements, adjacent ones of which are so constituted as to emit light of different primary colors. These elements are preferably so small and so closely spaced that the human eye is unable to resolve them at normal viewing distances. Consequently, differently colored light emitted from different adjacent elements will appear to blend, and the impression of any desired coloration can be created merely by control of the relative intensities with which light is emitted from the different elements. While this system for reproducing colored images appears fundamentally simple, it has been found that certain practical difficulties stand in the way of achieving completely faithful color reproduction by means thereof.

In the first place, the different phosphors, which are suitable for use in the formation of the aforementioned minute elements emissive of light of different primary colors, normally have widely differing inherent light emission efiiciencies. By this is .meant that they transform the energy of the electron beam into light energy with different efficiencies. Assuming now that the different minute screen elements are formed of these phosphor materials in their preferred manner, namely in substantially identical geometrical configurations, then it will be apparent that beam intensity control by the aforedescribed received signal will result in production of light of the wrong color. Consider, for example, the case in which white light emission from the screen is desired. In that situation, all the color representative portions of the received signal which is used to control the electron beam intensity are of equal amplitude, thereby indicating that the primary colors exist with equal intensities in the televised scene. However, the screen elements made of the most efiicient phosphor will emit more light, under impingement by this beam, than the elements made of less efficient phosphors. Consequently the light emitted from the screen will not appear to be white but will have the hue of the most efficient phosphor.

Likewise, when it is desired to produce light which is predominantly of a given color, the electron beam is controlled in such manner that, at impingement upon different screen elements, its intensities are in the same ratio as the amounts of light whose emission it is desired to obtain from these different elements. The amounts of light which are actually produced, however, will not be in this ratio, but will be in a dilferent ratio owing to the different inherent light emission efficiencies of the different phosphors, and an improperly colored image will be produced.

Various solutions to this problem have been proposed heretofore. One of these involves the generation of three separate electron beams in the cathode ray tube or, at least, the generation of a beam having three separate portions whose intensities can be separately controlled. From the received signal there are then derived three separate signals, each one of which is representative only of intelligence concerning a single one of the primary colors. Each of the three electron beams of the cathode ray tube is then controlled in intensity by a derived signal representative of a different color intelligence. Finally, means are provided, such as, for example, a masking plate with suitably placed apertures, for insuring that each beam can impinge only upon screen elements emissive of light of the color which the signal controlling the channels through which the intensity control signals are applied to control the three separate electron beams it is then possible to vary the relative intensities of these beams, independently of color intelligence, in such a manner as to compensate for diiferences in light emission efficiencies among the dilferent phosphors. It is apparent that such a scheme involves apparatus of considerable complexity and costliness, for it is exceedingly difficult to manufacture a cathode ray tube with means for producing three beams and to provide the necessary circuits for deriving the separate color representative signals from the received signal.

Another approach to the same problem involves increasing the total area of the screen occupied by phosphor elements emissive of light with comparatively low efficiency, relative to the total area of the screen which is occupied by phosphor elements emissive of light with comparatively high efficiency. For this purpose, the elements made of phosphors with low inherent light emission efficiency are either extended in size relative to the elements made of high efliciency phosphor, or the former are made more numerous than the latter. This approach to the problem has also proved unsuccessful because any increase in the size or number of the screen elements emissive of light of a particular color also increases the time during which the electron beam dwells upon these elements. Since the intervals during which the received signal is representative of intelligence concerning a particular color are of extremely short duration, such an increase in the interval during which the beam, whose intensity is controlled by this signal, impinges upon elements emissive of light of any particular color will cause the light emission therefrom to be controlled in part in accordance with intelligence concerning other colors or even no colors at all. This will result in desaturation and/or hue distortion of the reproduced image.

Another problem which arises in the operation of a color television receiver, whose cathode ray tube screen structure is constituted of the aforedescrioed minute elements emissive of light of different colors and whose beam intensity is controlled by a signal having successive portions representative of different color intelligence, is that of maintaining accurate registry between the instants at which the electron beam is impingent upon a screen element emissive of light of a particular color ticular system for securing such registry of beam im- -pingernent. cathode ray tube comprises, in addition to the afore- In that system the screen structure of the mentioned minute fluorescent elementsneeded to reproduce the televised image, also a plurality of so-called indexing elements provided specifically for use in obtaining beam registry. These elements are disposed over certain portions of the screen area (notably over the gaps between phosphor elements) in a regular pattern relative to the fluorescent elements, and are characterized in that the material of which they are constituted has a secondary electron emission ratio which differs substantially from that of the remaining area of the screen. As the beam traces its normal scanning raster on the screen, it passes over successive ones of these indexing elements, and the total secondary electron emission current from the screen fluctuates in accordance with the differences in secondary emission ratios of the different regions of beam impingement. This secondary emission current is then collected and its fluctuations are interpreted as indications of beam impingement upon predetermined portions of the screen. As set forth in the aforementioned copending application, these fluctuations in secondary emission current from the tube may then be variously utilized to carry out the broad objectives of effecting synchronism between beam impingement on a colored light emissive screen element and beam modulation in accordance with information respecting the same color. This may be done either by utilizing the current fluctuations to control the times and rate of application of such information to the beam intensity control grid, or by utilizing them to control the rate of beam deflection across the screen. The former mode of utilizing the secondary emission current variations is particularly described in the aforementioned Bocciarelli application, while the latter is more fully discussed in the copending U. S. patent application of Wilson P. Boothroyd, Serial No. 219,093, filed April 3, 1951 and assigned to the assignee of the present invention. For the purposes of the present invention, the particular manner of utilizing these indexing signals is immaterial and therefore, need not be further discussed at this point.

Heretofore the indexing elements provided for the aforementioned reasons have always been separate and distinct from the fluorescent elements of the screen structure. Specifically they have consisted of appropriately arrayed and configured elements of a material such as gold, for example, whose secondary electron emission ratio is substantially different from that of the phosphors of which other regions of the cathode ray tube screen structure are commonly made. Sometimes, in order to enhance discrepancies in secondary emission ratio between the indexing elements and other regions of the screen area, the entire interior surface of the fluorescent elements of the screen structure was coated with a thin electron permeable layer of aluminum, the indexing elements being superimposed upon this aluminum layer rather than directly upon the fluorescent elements. In any event, the provision of indexing elements which were separate and distinct from the fluorescent elements, and sometimes even spaced therefrom by the aforementioned aluminum layer, necessitated a plurality of manufacturing operations which greatly increased the difficulties involved in construction of such screen structures. It was particularly difficult to maintain accurate registry between the positions of fluorescent elements and those of the indexing elements.

It is, accordingly, the primary object of the invention to provide an improved cathode ray tube screen structure.

It is another object of the invention to provide an improved cathode ray tube screen structure which is adapted for the faithful reproduction of black-and-white, and also of colored images.

It is another object of the invention to provide a cathode ray tube screen structure having different regions responsive to impingement by an electron beam with predetermined relative intensities to emit light of different primary colors with substantially the same said relative intensities.

It is still another object of the invention to provide an improved cathode ray tube screen structure having different regions emissive of light of different colors with substantially equal intensities in response to impingement by an electron beam of substantially uniform intensity.

Still another object of the invention is the provision of a cathode ray tube screen structure having different regions responsive to impingement by an electron beam of substantially uniform intensity to emit light of different primary colors in relative amounts suflicient to create, in the aggregate, the sensation of white light emission.

Still another object of the invention is the provision of a cathode ray tube screen structure having different regions responsive to impingement by an electron beam of uniform intensity to emit light of complementary primary colors with substantially equal intensities and to emit secondary electrons with substantially different intensities.

It is a still further object of the invention to provide an improved cathode ray tube screen structure having predetermined regions whose secondary electron emission ratio differs substantially from that of other regions of the screen.

It is still another object of the invention to provide a cathode ray tube screen structure having indexing means whose registry with predetermined fluorescent elements is inherently assured.

It is a further object of the invention to provide a cathode ray tube screen structure equipped with indexing means, in whose manufacture no special precautions need be taken to insure registry between the indexing means and fluorescent elements emissive of light of a predetermined color.

The foregoing objects, as well as others which will appear, are achieved in accordance with the invention by providing a cathode ray tube screen structure which is constituted of minute elements made of different phosphor materials which are emissive of light of different colors in response to electron beam impingement. This screen structure is characterized by the fact that those phosphors, used in certain screen elements, which have higher light emission efficiencies than the phosphors used in other screen elements are mixed with additional material having substantially lower light emission efficiency in such proportions that the light emissivity of the elements constituted of this mixture is substantially the same as that of the elements which are constituted only of phosphors of lower light emission efficiency. A screen structure of this kind will be effective to reproduce an image in true colors when scanned by an electron beam whose intensity is controlled in accordance with the different primary colors at appropriately time spaced intervals.

It is desired. to emphasize at this point that the light emission efficiency of any material, e. g. a phosphor, is an inherent property of that phosphor and is independent of the physical configuration of the screen elements which may be made of this phosphor. This light emission efficiency is not to be confused with the light emissivity of the screen elements, which is the amount of light emitted under a stimulus of reference intensity and is a function, not only of the light emission efficiency of the materials of which they are constituted, but also of the physical dimensions of theeleme'nts, and of the distribution of so-called color centers within these elements.

If, in a cathode ray tube screen structure, all the screen elements have the same physical configurations, then their light emissivities will be equal if the light emission efficiencies of their respective constituents are also equal. In any case, however, it is the light emissivities of the elements which it is desired to equalize, since the eye responds to the total amount of light emitted from the screen elements, and this total amount is determined, as has been indicated, not only by the phosphor efllciencies butralso by the physical configurations of the elements.

We have also found that the indexing function, which is frequently required of screen structures of the type under consideration in order to insure synchronism between beam impingement upon phosphor elements emissive of light of a particular color and instants when the beam intensity is controlled in accordance with information respecting this color, can be performed simultaneously by this screen structure. To this end the material of low light emission efliciency, which is mixed with certain phosphor or phosphors, as hereinbefore described, in order to reduce the light emissivity of the elements in which they are incorporated, is also a material which has a secondary emission ratio substantially greater than that of the phosphor materials of which other screen elements are constituted. Materials which have relatively low light emission efficiency, but which have a relatively high secondary emission ratio, are readily available.

By the use of a screen structure of the kind herein before briefly described, it becomes possible to realize a system in which the electron beam may be controlled in intensity in accordance with different color intelligence at different intervals, and in which an image in full color will be reproduced on the screen scanned by the beam.

The invention will more readily be understood from a consideration of the following detailed description with reference to the accompanying drawing, in which:

Figure 1 shows a color television receiver system embodying the invention; and

Figure 2 shows, by means of a greatly enlarged fragmentary view, details of the screen structure of the cathode ray tube illustrated in Figure 1.

In Figure l of the drawing, to which more particular reference may now be had, there is illustrated a cathode ray tube which includes many conventional elements such as electron emissive cathode 11, electron beam intensity control grid 12, focus coil 13, horizontal and vertical deflection coils 14 and accelerating anode 15. In fact, with the exception of its screen 16, all the elements of this cathode ray tube may be entirely conventional so that they do not require detailed description here. There is also provided a conventional source of focus current 17 which is connected to focus coil 13 for the purpose of causing the beam to impinge upon the screen 16 in a small region or spot. Deflection coils 14 are connected to conventional horizontal and vertical deflection circuits 18 and operate to deflect the beam repetitively across the cathode ray tube screen so as to trace thereon a conventional raster. The accelerating anode 15, which will preferably take the form of a deposit of conductive carbon particles encircling the inside of the flared portion of the cathode ray tube envelope near to, but spaced and insulated from the screen, is connected to a conventional source of anode potential A+-]-, while the screen itself is connected to a source of screen potential A]- through a resistor 19. The beam intensity control grid 12 is connected to three signal modulators 2t}, 21 and 22 which are, in turn, connected to signal input terminals 23, 24 and 25, respectively. Screen 16 is connected to an amplifier 26 through a DC. blocking capacitor 27, the output of the amplifier being in turn connected to a phase shifter 28. The latter is provided with three output terminals, designated 29, 30 and 31 and respectively connected to signal modulators 20, 21 and 22. In operation, a beam 6 of :electrons is emitted from cathode 11, focused by coil 13 and projected toward the screen by accelerating anode 15. The deflection coils 14 produce repetitive deflection of this beam across the screen so as to form, in the conventional arrangement, spaced horizontal scanning lines thereon. The intensity of this beam of electrons is controlled by the video signal supplied to grid 12 from signal modulators 20, 2'1 and 22.

For this purpose, color signal input terminals 23, 24 and 25 are supplied from a television receiver with separate signals indicative of the red, green and blue components of the televised scene, respectively, which signals preferably have had their D.-C. components restored. The system then operates to modulate three sine waves in accordance with these three signals, these sine waves being mutually displaced in such a manner that first the wave modulated with the red video signal reaches its peak, then the wave modulated with thegreen video signal and finally the wave modulated with the 'blue signal. By virtue of this arrangement, the intensity of the cathode ray beam in tube 10 is controlled first by the red video signal, then by the green signal, and finally by the blue signal, and so on recurrently.

The particular manner in which this modulation may be effected and controlled is explained in detail hereinafter. First, however, the details of construction which enable the screen to reproduce this color information in visible form will be considered. These are all illustrated in Figure 2 of the drawing, to which more particular reference may now be had. As shown therein, screen 16 is comprised of a substrate 32, of a solid transparent material such as glass, upon which is preferably deposited a thin film 33 of some electrically conductive transparent material such as stannic oxide.

deposited a number of narrow phosphor strips 34, 35 and .36 extending vertically across the screen surface. As. shown in Fig. 2, these strips may be spaced apart from:

each other by substantial distances. They are constituted of different phosphors so that strips 34 are responsive to electron beam impingement to emit red light, while strips 35 and 36 are similarly responsive to emit green and blue light, respectively. It will be readily understood that it is desirable, in the interests of fidelity of color reproduction, that light of the different component colors represent-- 'ative of a given picture element emanate, as nearly as. possible, from the same region of the screen area. There-- fore it is the usual practice to one strip or" materials which emit light of one of the primary colors, while mak-- ing the two adjacent strips of materials which emit light of the other two colors, respectively. This consideration,

together with the order of occurrence of colors representative signal portions produced by the sampling sematerials constituting strips emissive of red and blue light. Strips in which such additional material is incorporated constitute the indexing elements of our screen structure at the same time that they continue to perform their functions as fluorescent elements.

Thus, as the beam traces its scanning raster upon the screen structure 16 of Figures 1 and 2, it will not only produce light emission from the impinged fluorescent ele-- ments but, in addition, each time the beam strikes a green. strip 35, it will also cause a change in current flow through. screen resistor 19. The attendant change in screen poten-- tial will be transmitted as an indexing signal throughcapacitor 27 to amplifier 2.6 and thence to phase shifter 28. This phase shifter is so arranged that, upon being supplied with an indexing signal which is indicative of On the electron beam. confronting side of this film 33, there are additionally '29, 30 and 31 respectively, these signals being so phased with respect to the indexing signal as to peak at the times when the beam is impingent upon correspondingly colored strips 34, 35 and 36, respectively. These signals are then supplied, through the previously mentioned connections, to the signal modulators where their amplitudes are controlled by the three different color signals as hereinbefore explained. In practice each modulator may comprise a multi-grid vacuum tube to one of whose grids the appropriate color signal is continuously applied while its other grid is supplied with the appropriately phased sine wave from phase shifter 28.

Thus it is seen that the entire indexing system associated with a screen structure embodying our invention operates as a servo loop, beam impingement upon indexing elements of the screen structure assuring the supply of appropriate color information to the cathode ray tube grid for the maintenance of accurate color reproduction. As has been pointed out, the particular method of effecting this control is immaterial for the purposes of our invention, the practical modulator arrangement herein de scribed being merely illustrative. Note, however, the importance of incorporating the indexing means directly into the fluorescent elements, with the result that the occurrence of the indexing signal is always positively indicative of beam impingement on say, a green light emissive strip, thus circumventing any registry problems due to misalignment of separate fluorescent and indexing elements. The problem of color registry is at best so diflicult and complex that the complete elimination of this source of error in accordance with our invention represents an important advance in the art.

So far as the secondary electron emissive characteristics of our new screen structure are concerned, it is immaterial what specific phosphors are selected to make up the fluorescent strips of which the screen is normally constituted, and it is also immaterial what particular materials are added to some of these strips to cause them to function as indexing elements, provided only that these added materials meet the aforementioned requirements respecting their secondary emission ratios. In practice, however, other considerations become important, too. For example, the added materials are preferably chemically inactive .so that they do not produce chemical changes in the phosphor which might impair its inherent light emissivity either with respect to brightness or coloration. The added materials are preferably readily miscible with the phosphor prior to deposition and are preferably adapted to be deposited by the same methods as the phosphor. Furthermore it will be recalled that another aspect of the inven- :tion is concerned with the provision of a screen structure capable of emitting light of the different primary colors in substantially equal amounts under impingement by an electron beam of constant intensity. Yet a given area of the phosphor (zinc orthosilicate), which is most commonly used for the emission of green light from such a screen, is inherently capable of producing more than twice as much visible green light as the total amount of visible blue or red light produced by an equal area of the commonly used blue or red phosphors (calcium magnesium silicate and zinc phosphate). Consequently, it is preferred to place the added materials, which are not themselves light emissive, in the strips made of green light emissive phosphor so as to reduce their relative light emissivity at the same time that their relative electron emissivity is enhanced. With red, green and blue light emissive phosphor strips of approximately equal beam exposed areas, we have found that the formation of the green strips with approximately equal proportions of phosphor and added material produces the best balance between the light emissivities of the different color strips. It will be understood that changes in the relative sizes of these strips will necessitate changes in the proportions of the ingredients,'as will also the use of phosphors of differently related inherent light emission efficiencies. Furthermore, it will be understood that if it is important for any reason to keep the amount of added secondary electron emissive material below that value which is required for color balance, then additional amounts of a material, such as unactivated willemite or silicon dioxide, having relatively low secondary emissivity but having all the other characteristics required of the added material, may be incorporated in the appropriate strips. In fact, even if indexing performance is of no importance at all, so that no secondary electron emissive material need be used, color balance can still be secured by adding only non-emissive materials to the strips made of the most efficient phosphor. It will be understood that, while it is preferred to mix with the most efficient phosphor a material which is not fluorescent to any appreciable extent, our invention also contemplates the use of mixing materials which do have some appreciable fluorescence, the only essential requirement being that the light emission efficiency of the mixing material be lower than that of the less efiicient phosphor materials, else color balance cannot be obtained by its use.

It should also be noted that the aforedescribed incorporation of additional material into certain of the phosphors, so as to change their net effective secondary electron emissivity and/or colored light emissivity, is done by mechanical mixture and preferably does not involve chemical reactions. Within the contemplation of the invention, the added material may either be distributed uniformly throughout the phosphor matrix or may be nonuniformly distributed so that there exists a higher concentration thereof in one portion of the phosphor matrix than in others. A uniform distribution is advantageous since it permits the use of the simplest methods of screen fabrication in which the mixture is prepared in advance and then deposited on the glass substrate by any conventional process. However, a non-uniform distribution may also be advantageous under certain circumstances. Thus, if the concentration of added material is made higher near the electron beam confronting surface of the phosphor strips than elsewhere, strong secondary electron emission will be produced since the beam will be of maximum intensity when it encounters the high concentration of secondary electron emissive particles. Therefore, in this case, the proportion of added material may be somewhat less than in the case where the distribution is uniform.

It should be borne in mind also that different phosphors may possess inherently different secondary electron emissivities. Thus in the red, green and blue striped tube here illustrated the green phosphor may, for example, have the lowest secondary electron emission ratio. Nevertheless, its inherent light emission efficiency is usually so much superior to that of the other phosphors that it may still be desirable to incorporate the added secondary emitter material therein, for the sake of obtaining improved color balance. If that consideration prevails, the distribution of the added material within the phosphor will of course have to be denser, in order to produce the same difference in secondary emission ratio between indexing and non-indexing elements, than if the same material had been added to a phosphor with a higher secondary electron emission ratio. In any case, this distribution should be such that the net effective secondary electron emissivity of the indexing portion, or the combined electron emissivity of the phosphor and of the added material, is different from that of the other portions of the screen.

The particular blue and red phosphors hereinbefore mentioned as suitable for use in a screen structure embodying the invention have inherent light emission efiiciencies which are so nearly the same that no special precautions need be taken to insure equality of light emission therefrom. However, there are available other blue and red phosphors with substantially different efficiencies. If.

these are used in the formation of the screen structure, then it will also be necessary to incorporate a certain amount of non-fluorese'cent material in the more efficient of these phosphors in order to reestablish color balance between them. Since we know of no blue or red phosphorwhose efficiency is as great as that of the aforementioned green phosphor, the amounts of material added to the blue or red phosphor to reduce its relative efliciency will, in no presently known case, exceed the amount added to the green phosphor. The actual proportions can,.of course, be readily determined once specific phosphors have been selected for use.

The aforementioned phosphor materials all have low secondary electron emission ratios, particularly at the high screen potentials common in cathode ray tubes. Therefore the materials incorporated therewith for indexing purposes may be chosen from the number of good secondary electron emitters. A variety of such materials are known, including magnesium oxide, gold, tungsten and others.

As has been pointed out, since the green phosphor is the most efficient of the phosphors named above, the added material is usually incorporated with this green phosphor. In a uniform mixture, we have used, with good success, green phosphor strips containing 25 to 75 percent by weight of'magnesium oxide powder, for example. Similarly, other materials such as gold or tungsten, are preferably mixed with the appropriate phosphor in powdered form. The actual proportions will vary, of course, depending upon the actual materials used and depending upon the strength of the indexing signal which it is desired to obtain from the screen. Given the particular requirements, the proper proportions can readily be ascertained by those skilled in the art.

In depositing the phosphor strips on their base material any one of several methods can be used. These include silk screening, settling, and spraying. In practicing our invention we prefer to use spraying which is carried out by spraying the entire phosphor strip base successively with the mixtures of the different phosphors, one of which has had magnesium oxide, for example, added thereto, as Well as polyvinyl alcohol and ammonium dichromate. Between applications of different phosphors, the previously deposited layer is exposed to light through narrow slits fo'rmedin an otherwise opaque mask at the regions where phosphor strips are to be formed. This exposure fixesthe desired portions of the phosphor and the remaining portions can then be washed off, leaving the strips on the screen. This procedure is repeated for each different set of strips, either with different masks or by changing the alignment of a single mask. Very accurate and sharply defined strips can be deposited by this method.

In view of the foregoing description of our invention and the preferred practice thereof, it will be apparent that modifications thereof will occur to those skilled in the art without departing from our inventive concept. Accordingly, we desire the latter to be limited only by the appended claims.

We claim:

1. A screen structure for a cathode ray tube, said screen structure comprising a first portion constituted of fluorescent material having predetermined light emission efficiency and having predetermined secondary electron emission ratio, and a second portion constituted of a mixture of fluorescent material having greater light emission efficiency and non-fluorescent material, the relative proportions of said fluorescent material and of said other material being such that the light emissivity of said second portion is substantially the same as that of said first portion and said non-fluorescent material having a secondary electron emission ratio relative to that of the fluorescent material constituting said first portion such that the net effective secondary electron emissivity of said second portion is substantially different from that of said first portion.

2. A screen structure for a cathode ray tube, said screen structure comprising a first portion constituted it of fluorescent material having predetermined light emission efficiency and having predetermined secondary electron emission ratio and a second portion constituted of a mixture of fluorescent material having greater light emission efficiency and material having a secondary electron emission ratio substantially different from that of said fluorescent material constituting said first portion, the relative proportions of said fluorescent material and of said other material in said second portion being such that the net effective secondary electron emissivity of said second portion is substantially different from that of said first portion and the light emission efliciency ofsaid other material relative to that of said fiuorescent material constituting said first portion being such that the light emissivity of said second portion is substantially the same as that of said first portion.

3. A structure according to claim 2 characterized in that said other material is magnesium oxide.

4. A screen structure for a cathode ray tube, said screen structure comprising a first portion constituted of calcium magnesium silicate, a second portion constituted of zinc phosphate and a third portion constituted of a mixture of zinc orthosilicate and magnesium oxide, the relative proportions of Zinc orthosilicate and magnesium oxide in said mixture being such that the light emissivity of said third portion is substantially the same as that of said first and second portions.

5. A colored light emissive screen structure for a cathode' ray tube, said screen structure comprising first, second and third portions of substantially equal surface areas, said first portion being constituted of calcium magnesium silicate, said second portion being constituted of zinc phosphate and said third portion being constituted of a mixture of zinc orthosilicate and magnesium oxide in substantially equal proportions.

6. A screen structure for a cathode ray tube, said structure'comprising different portions having different secondary electron emissivities, one of said portions being constituted of a fluorescent material, and another of said portions being constituted of a mixture of fluorescent material and another material having a secondary electron emission ratio substantially different from that of said fluorescent material constituting said one portion, the relative distribution within said mixture of said fluorescent material and of said other material being such that the net effective secondary electron emissivity of said other portion is substantially different from that of said one portion.

7. A screen structure for a cathode ray tube, said structure comprising different portions having different secondary electron emission ratios, one of said portions being constituted of fluorescent material and another of said portions being constituted of a mixture of fluorescent material and another material having a secondary electron emission ratio substantially different from that of the fluorescent material constituting said one portion. said mixture of fluorescent material and other material being in such proportions that the net effective secondary electron emission ratio of said other portion is substantially different from that of said one portion.

8; A screen structure for cathode ray tubes, said structure comprising different portions having different secondary electron emissivities, one of said portions being constituted of a fluorescent material, and another of said portions being constituted of a mixture of fluorescent material and another material having a secondary election emission ratio substantially great than that of said fluorescent material constituting said one portion, the relative distribution within said mixture of said fluorescent material and of said other material being such that the net secondary electron emissivity of said other portion is substantially greater than that of said one portion.

9. A screen structure for cathode ray tubes, said structure comprising: a first portion constituted of a fluorescent material having predetermined secondary electron emissivity; a second portion constituted of an other fluorescent material having predetermined secondary electron emissivity; and a third portion constituted of a mixture of a third fluorescent material of predetermined secondary electron emissivity and another material having a secondary electron emissivity substantially greater than that of any of said fluorescent materials, the relative distribution of said fluorescent material and said other material Within said mixture being such that the net effective secondary electron emissivity of said third portion is substantially greater than that of said first and second portions.

10. A screen structure for cathode ray tubes, said structure comprising a first portion having predetermined secondary electron emissivity, said first portion being constituted of a fluorescent material, and a second portion disposed in predetermined geometrical relationship with respect to said first portion, said second portion being constituted of a mixture of fluorescent material and another material having a secondary electron emission ratio substantially different from that of said fluorescent material constituting said first portion, the relative distribution within said mixture of said fluorescent material and of said other material being such that the'net effective secondary electron emissivity of said second portion is substantially different from that of said first portion.

11. A screen structure for cathode ray tubes, said structure comprising a plurality of parallel elongated strip-like portions, certain of said portions being constituted of a fluorescent material having predetermined secondary electron emissivity, and others of said portions being constituted of a mixture of fluorescent material and another material having a secondary electron emission ratio substantially different from that of said fluorescent material constituting said certain portions, the relative distribution Within said mixture of said fluorescent material and of said other material being such that the net effective secondary electron emissivity of said other portions is substantially different from that of said certain portlons.

12. A screen structure for cathode ray tubes, said structure comprising a plurality of strip-like portions, different ones of said portions including red, green and blue light emissive phosphors, respectively, portions emissive of light of one of said colors also including another material in mixture with the phosphor of said lastnamed portions, said other material having a secondary electron emission ratio substantially greater than that of said light emissive phosphors, the relative distribution in said mixture of said phosphor and said other material being such that the net effective secondary electron emissivity of said mixture is substantially greater than that of portions emissive of light of other colors.

13. A screen structure for cathode ray tubes, said screen structure comprising a plurality of laterally displaced parallel strips, adjacent ones of said strips being constituted of materials responsive to impingement by an electron beam to emit light of different colors and predetermined ones of said strips including, in mixture with the light emissive materials of said strips, magnesium oxide in such distribution that the net effective secondary electron emissivity of said predetermined strips is substantially greater than that of other strips.

14. A screen structure for cathode ray tubes, said screen structure comprising a plurality of laterally displaced parallel strips, certain ones of said strips being constituted of fluorescent materials, and others of said strips, disposed in a predetermined pattern with respect to said certain strips, being constituted of a mixture of a fluorescent material and of magnesium oxide, the proportion of magnesium oxide in said mixture being in the range of from twenty-five to seventy-five percent by weight. I

15. A screen structure for cathode ray tubes, said screen structure comprising a plurality of laterally displaced parallel strips, certain of said strips being constituted of fluorescent materials, and others of said strips, disposed in a predetermined pattern with respect to said certain strips, being constituted of a mixture of a fluorescent material and of tungsten in such proportions that the net effective secondary electron emissivity of said other strips is substantially greater than that of said certain strips.

16. A screen structure for a cathode ray tube, said structure comprising different portions having different secondary electron emissivities, one of said portions being constituted of a nonfluorescent material, and another of said portions being constituted of a mixture of fluorescent material and another material having a secondary electron emission ratio substantially different from that of said fluorescent material, the relative distribution within said mixture of said fluorescent material and said other material being such that the net effective secondary electron emissivity of said other portion is substantially different from that of said one portion.

17. A screen structure for a cathode ray tube, said structure comprising a plurality of different portions having different secondary electron emissivities, one of said portions being constituted of a non-fluorescent material, a second of said portions being constituted of a fluorescent material and a third of said portions being constituted of a mixture of fluorescent material and another material having a secondary electron emission ratio substantially different from that of said last-named fluorescent material, the relative distribution within said mixture of said fluorescent material and said other material being such that the net effective secondary electron emissivity of said third portion is substantially different from that of either of said first and second portions.

18. A screen structure for a cathode ray tube, said structure comprising different portions having different secondary electron emissivities, one of said portions being constituted of a material having predetermined secondary electron emission ratio, and another of said portions being constituted of a mixture of fluorescent material and another material having a secondary electron emission ratio substantially different from that of said fluorescent material, the relative distribution Within said mixture of said fluorescent material and of said other material being such that the net effective secondary electron emissivity of said other portion is substantially different from that of said one portion.

19. A screen structure for a cathode ray tube, said structure comprising different portions having different secondary electron emissivities, one of said portions being constituted of a material having a predetermined secondary electron emission ratio, and another of said portions being constituted of a mixture of fluorescent material having a predetermined secondary electron emission ratio and another material having a secondary electron emission ratio substantially different from that of said fluorescent material, the relative distribution within said mixture of said fluorescent material and of said other material being such that the net effective secondary electron emissivity of said other portion differs from that of said one portion by an amount which is greater than the difference between the said predetermined secondary electron emission ratios of said material constituting said one portion and said fluorescent material.

20. A television picture tube comprising a layer of transparent electrically conducting material, a plurality of strips of phosphor coated on said layer, some of said strips having mixed therewith materials for causing their secondary emission coefficients to be different from the secondary emission coefficients of other of said strips.

21. A fluorescent screen structure for cathode ray tubes, said screen structure having a first region constituted of a first fluorescent material emissive of light of a given color and having a second region constituted of a mixture of a second fluorescent material emissive of light of a diflerent color and of substantially non-fluorescent material, said second fluorescent material having greater light emission efliciency than said first fluorescent material.

22. The screen structure of claim 21 further characterized in that said second fluorescent material and said non-fluorescent material are present in said mixture in such proportions that the light emissivity of said second region is substantially the same as that of said first region.

23. A fluorescent screen structure for cathode ray tubes, said screen structure having a first region constituted of a first material emissive of light of a given color, and having second and third regions, respectively constituted of a mixture of a second fluorescent material and substantially non-fluorescent material and of a mixture of a third fluorescent material and substantially non-fluorescent ma terial, said second and third' fluorescent materials being emissive of light in colors which are complementary to said given color and having greater light emission efliciencies than said first fluorescent material, and said materials being present in said mixtures in such proportions that the light emissivities of said second and third regions are substantially the same as that of said first region.

24. A fluorescent screen structure for cathode ray tubes, said screen structure having a first region constituted of a first fluorescent material emissive of light of one given color, and having second and third regions, respectively constituted of a mixture of a second fluorescent material and substantially non-fluorescent material and of a mixture of a third fluorescent material and substantially non- 14 fluorescent material, said second and third fluorescent materials being emissive of light in colors which are complementary to said given color, and said materials being present in each of said mixtures in such proportions that said first, second and third regions respond to equal electron beam excitation to emit white light.

25. A screen structure for cathode ray tubes, said screen structure comprising a surface having a plurality of laterally displaced regions, one of said regions being constituted of a first fluorescent material emissive of light of a given color and another of said regions being constituted of a mixture of a second fluorescent material emissive of light of a different color and of substantially nonfluorescent material, said second fluorescent material having greater light emission efficiency than said first fluorescent material and said materials being present in said mixture in such proportions that the light emissivity of said other region is substantially the same as that of said one region.

26. The screen structure of claim 25 further characterized in that said fluorescent regions occupy different areas of a substantially transparent substrate.

References Cited in the file of this patent UNITED STATES PATENTS 1,988,605 Michelssen Jan. 22, 1935 2,169,046 Headrick Aug. 8, 1939 2,242,644 De Boer May 20, 1941 2,243,828 Leverenz May 27, 1941 2,630,548 Muller Mar. 3, 1953 2,631,259 Nicoll Mar. 10, 1953 2,654,675 Thierfelder Oct. 6, 1953 

